The strong relationship between nitrogen availability in soil and crop yields is well known in the art. Of the major plant nutrients (N, P, K, Ca, Mg, and S), nitrogen is the nutrient required in the greatest quantities and much research has been conducted over the years to determine the nitrogen fertilizer requirements of crops. Applying too little nitrogen causes severe economic losses by reducing crop yields, while applying too much nitrogen is not only expensive but also causes harmful levels of NO3 to leach into the ground and surface water supplies.
Determining the need for nitrogen is particularly difficult since most soil nitrogen is tied up in the soil organic matter and is unavailable to plants. The amount of nitrogen that is made available (in the form of NO3 and NH4) to plants during the growing season depends strongly on factors such as the previous cropping history of the field, soil organic matter content, microbial activity and rainfall (which quickly leaches the available forms of nitrogen away from the root zone). These difficulties have led some investigators to explore the use of plant nitrogen analysis instead of soil tests, since the plant itself should be a reliable indicator of the amount of nitrogen available to it.
Chemical assaying of plant nitrogen status is typically a slow and time consuming process because, as in the case of corn, about a dozen leaves are usually collected which must then be dried, ground, and analyzed using one of several laboratory techniques. Interpreting the laboratory data involves considerable uncertainty because leaf nitrogen concentrations decrease during the growing season and nitrogen concentrations further depend on which leaf is sampled. These difficulties led to efforts to develop rapid, non-destructive methods for assessing the nitrogen status of crops.
Crops that are well-supplied with nitrogen grow vigorously and look deep green in color due to their high leaf chlorophyll concentrations, while crops deficient in nitrogen grow less well and look yellow-green (chlorotic) due to low concentrations of chlorophyll. This led to the development of a device for measuring leaf chlorophyll content, called a chlorophyll meter, which measures light transmittance through individual leaves in the red (approximately 670 nm) and near-infrared (approximately 800 nm) regions of the electromagnetic spectrum. Furthermore, above a certain level of nitrogen fertilizer application, crop growth and chlorophyll content reach a plateau or a maximum, referred to hereinafter as the nitrogen fertilizer response plateau. This permits the use of a reference strip which provides a baseline against which nitrogen stress can be measured. However, due to the fact that a very great number of leaf measurements must be taken in order to survey an entire field for variable rate fertilizer adjustments, this method has not been widely accepted in commercial agriculture.
An additional problem with leaf chlorophyll or leaf color measurements is that the difference in leaf chlorophyll (or color) between a nitrogen deficient plant and a nitrogen sufficient plant is relatively small. By the time nitrogen fertilizer deficiency has caused a detectable change in leaf color, significant and unrecoverable yield reductions have already taken place. Therefore, leaf chlorophyll measurements using leaf transmittance have to be very precise, requiring measurements to be done under controlled illumination and light-path geometry. Measurements must be done on individual leaves clamped within an instrument. It is not possible at present to measure the entire crop canopy chemical attributes (color, chlorophyll or nitrogen concentration) with sufficient precision to be of value for detecting nitrogen deficiencies before irretrievable losses have already taken place. Measuring entire crop canopy reflectance in the green band does not measure leaf greenness (i.e. color), it merely measures the reflection in the green band of contrasting targets within the field of view of the sensor (plants, soil and shadows). Using chlorophyll fluorescence is even more problematic because fluorescence is not only a function of nitrogen status and leaf chlorophyll content but also depends on the short-term changes in the plant's metabolic pathways. Fluorescence is a very unstable physiological parameter which varies from hour to hour with illumination, drought and a variety of other stresses. It is difficult to see how it could be used for determining the nitrogen needs of a field crop.
A more sensitive alternative to measuring color change is to measure changes in standing crop biomass or some other physical attribute such as leaf area, crop density, crop cover, etc. Measuring the entire crop canopy physical attributes is radically different from and has several advantages over measuring individual leaf chemical attributes such as chlorophyll content or color. Changes in standing crop density, for example, show a greater sensitivity to nitrogen supply than do changes in leaf color. Also, the measurement is integrated over a larger area and is much easier to do in real time. It can, for example, be done from a tractor or center pivot using one of several rapid, non-destructive methods or from aircraft or satellites using crop reflectance.
Various rapid, non-destructive techniques for assessing standing crop biomass and other related canopy physical attributes such as leaf area index and percent ground cover have also been developed. Included among these is the measurement of canopy reflectance in the visible, near-infrared (NIR), and scattering in microwave regions of the electromagnetic spectrum. Other techniques for rapid, non-destructive estimation of crop standing biomass have included measurements of canopy electrical capacitance, attenuation of β-particles and other ionizing radiation, and measurements of crop canopy transmittance in certain regions of the solar spectrum (between 300 nm and 3000 nm).